Are toe‑brachial index (TBI) measurements accurate in patients with diabetes and chronic kidney disease?

Toe-Brachial Index Accuracy in Diabetes and CKD Patients

Yes, the toe-brachial index (TBI) is accurate and specifically recommended for patients with diabetes and chronic kidney disease (CKD), particularly when the ankle-brachial index (ABI) is unreliable due to noncompressible vessels. 1, 2

Why TBI is Superior in These Populations

Digital arteries are rarely noncompressible, making TBI the preferred test when tibial arteries become calcified and incompressible—a frequent occurrence in diabetes and CKD 1. This is critical because:

• Noncompressible tibial arteries occur frequently in diabetes and CKD, resulting in falsely elevated ABI values >1.4 even when significant PAD exists 1
• The ABI sensitivity is reduced in the presence of diabetes (69-79% sensitivity in symptomatic PAD patients) 1
• TBI measurements bypass the problem of medial arterial calcification that invalidates ABI readings 1, 2

When to Use TBI

Measuring TBI is recommended (Class I, Level C) in patients with diabetes or renal failure if resting ABI is normal 2. More specifically:

• Use TBI when ABI >1.4 (clearly noncompressible vessels) 1
• Consider TBI when ABI is normal (0.9-1.4) but clinical suspicion for PAD remains, especially in diabetes/CKD patients 1
• TBI should be measured with waveforms, TcPO₂, and/or SPP for comprehensive assessment 1

Diagnostic Thresholds

• TBI ≤0.70 is abnormal and diagnostic of PAD 1
• TBI >0.70 is normal 1

Evidence Supporting TBI Accuracy

Recent research demonstrates TBI's clinical utility:

• Strong correlation with ABI (Pearson correlation 0.82-0.83) in patients with compressible vessels, but TBI remains accurate when ABI fails due to calcification 3
• Superior validity parameters in diabetics: sensitivity 0.88, specificity 0.88, with excellent negative predictive value (0.97) 4
• Excellent interrater reliability (ICC 0.85-0.93) for toe pressure measurements in diabetes patients 5
• TBI was significantly lower in diabetic patients compared to controls (p=0.005), while ABI showed no difference (p=0.92), suggesting greater sensitivity for early disease 6

Important Caveats

The ABI-TBI difference does not provide additional diagnostic value beyond using TBI alone. One study found no evidence that diabetes creates larger discrepancies between ankle and toe pressures among patients with ABI ≤1.4 7. The key is recognizing when ABI is unreliable (>1.4 or normal despite clinical suspicion) and switching to TBI, not calculating differences between them.

Clinical implementation requires proper technique: Photoplethysmography (PPG) shows excellent reliability for toe pressure measurement (ICC 0.78-0.93) but requires trained personnel 5. The test is noninvasive and safe, making it suitable for widespread screening in at-risk populations 3.

Practical Algorithm

1. Measure resting ABI first in all patients with suspected PAD
2. If ABI >1.4: Proceed directly to TBI (noncompressible vessels confirmed)
3. If ABI 0.9-1.4 AND patient has diabetes/CKD: Strongly consider TBI, especially with clinical suspicion for PAD
4. If ABI <0.9: PAD confirmed; TBI can provide additional perfusion assessment but not required for diagnosis
5. Interpret TBI ≤0.70 as abnormal/diagnostic of PAD

The major pitfall is relying solely on ABI in diabetes and CKD patients, where up to 8-15% may have noncompressible vessels leading to missed diagnoses 3, 8. Current evidence shows only 46% of diabetic foot ulcer patients receive appropriate vascular screening 8, representing a significant gap in guideline adherence.